1. Field of the Invention
The present invention relates to a magnetic storage element, and more particularly to a magnetic storage element that stores data by a magnetoresistive effect.
2. Description of the Background Art
The magnetoresistive (MR) effect is a phenomenon in which electric resistance is changed by applying a magnetic field to a magnetic material, which effect is used for a magnetic field sensor, a magnetic head and the like. Recently, a nonvolatile magnetic random access memory (RAM) and a magnetic head using a conventional giant-magnetoresistance (GMR) effect as well as a tunneling magnetoresistance (TMR) effect that assures a still larger rate of change of resistance than the GMR effect have been investigated.
In the GMR element or the TMR element producing the GMR effect or the TMR effect, a so-called spin valve structure is known, in which a ferromagnetic layer/a non-magnetic layer/a ferromagnetic layer/an antiferromagnetic layer are stacked one on another, with the ferromagnetic layer and the antiferromagnetic layer being exchange-coupled to fix the magnetic moment of the relevant ferromagnetic layer, and spin is readily reversed only in the other ferromagnetic layer by an external magnetic field. In this case, spin can be reversed in one of the ferromagnetic layers with a small magnetic field, so that it is possible to provide a magnetoresistance element of high sensitivity. This magnetoresistance element is used for a high-density magnetic recording and readout head. In the GMR element, a metal film is used for the non-magnetic layer, while in the TMR element, a tunneling insulating film is used for the non-magnetic layer.
Investigations of application of the GMR element and the TMR element to the MRAM are shown, e.g., in Document 1 (S. Tehrani et al., “High density submicron magnetoresistive random access memory (invited)”, Journal of Applied Physics, vol. 85, No. 8, 15 Apr. 1999, pp. 5822-5827) and Document 2 (Naji et al., “A 256 kb 3.0V 1T1MTJ Nonvolatile Magnetoresistive RAM”, ISSCC 2001 Digest of Technical Papers, p. 122). When using the GMR element and the TMR element in the MRAM, these elements are arranged in a matrix, and a current is flown through a separately provided interconnection to apply the magnetic field. The two magnetic layers constituting each element are controlled parallel or antiparallel to each other, to thereby record data of “1” or “0”. Reading is performed utilizing the GMR effect or the TMR effect, utilizing the change in the element resistance value that depends on the parallel state or the antiparallel state of the magnetic layers.
The use of TMR elements in MRAM has primarily been investigated, since the TMR effect is more advantageous than the GMR effect from the standpoint of low power consumption. The MRAM utilizing the TMR elements has the MR change rate of not less than 20% at room temperature, and the resistance value in the tunnel junction is also large, so that a greater output voltage can be obtained. Further, spin reversal is unnecessary upon reading, meaning that a less current is required for the reading. With these features, the MRAM utilizing the TMR elements is expected to realize a nonvolatile semiconductor memory device consuming less power and allowing high-speed reading and writing.
As described above, in MRAM, data “1” or “0” is stored by switching the magnetization of one ferromagnetic layer in the TMR element. This ferromagnetic layer serving as the recording layer has a direction in which magnetization is easy (the low energy state) depending on the crystal structure or the shape. This direction is called an “easy axis”. In the state where the stored information is held, the ferromagnetic layer is magnetized in this direction. In contrast, the direction in which magnetization is difficult is called a “hard axis”.
The easy axis of the recording layer is normally determined by its shape, and corresponds to the longitudinal direction of the recording layer. As such, the magnetic field required for switching the magnetization of the recording layer, i.e., the switching field, changes depending on the shape of the recording layer. It is known that this switching field is approximately inversely proportional to the width of the recording layer and proportional to the thickness, as shown in Document 3 (E. Y. Chen et al., “Submicron spin valve magnetoresistive random access memory cell”, Journal of Applied Physics, vol. 81, No. 8, 15 Apr. 1997, pp. 3992-3994).
In MRAM, when cells are downsized for higher integration, the switching field would increase by the demagnetizing field, depending on the width of the recording layer. This means that a large magnetic field is required for writing, which leads to increased power consumption.
As a method for suppressing an increase of the switching field due to the downsizing as described above, a technique to eliminate the shape anisotropy of the recording layer is described in Document 4 (K. Inomata et al., “Size-independent spin switching field using synthetic antiferromagnets”, Applied Physics Letters, vol. 82, No. 16, 21 Apr. 2003, pp. 2667-2669) and Document 5 (N. Tezuka et al., “Magnetization reversal and domain structure of antiferromagnetically coupled submicron elements”, Journal of Applied Physics, vol. 93, No. 10, 15 May 2003, pp. 7441-7443). The recording layer according to this technique is shaped to have equal lengths in the easy axis direction and the hard axis direction. In this case, shape anisotropy is not obtained, and thus, the recording layer is made to have a stacked structure of ferromagnetic/non-ferromagnetic/ferromagnetic layers, and two ferromagnetic layers are coupled antiparallel to each other so as to control the magnetization distribution within the plane of the recording layer and to thereby provide magnetic anisotropy. The switching field of the recording layer with this configuration is approximated by the following expression (1):Hsw=2Ku(t2+t1)/|M2t2−M1t1|+4πC(k)|M2t2−M1t1|/w  (1)where Hsw represents the switching field of the recording layer, Ku represents anisotropic energy of the recording layer, t1 and t2 represent thicknesses of the respective ferromagnetic layers, and M1 and M2 represent saturation magnetizations of the respective ferromagnetic layers. Further, k represents an aspect ratio of the recording layer, and C(k) is a coefficient dependent thereon, and t and w represent thickness and width, respectively, of the recording layer.
C(k) can be regarded as “1” for the shape having an infinite length, and “0” for the isotropic shape. The first term in the right side of the expression (1) is a term by anisotropic energy, and the second term is a term describing the influence of the demagnetizing field generated by shape anisotropy. Herein, C(k)=0, and the influence of the demagnetizing field generated by the shape anisotropy can be ignored. As such, it is possible to suppress the increase in switching field due to the miniaturization of the recording layer.
In the above-described configuration, providing a difference between the products of saturation magnetization and thickness of the two ferromagnetic layers can decrease the magnetization switching field, as seen from the expression (1). However, it is reported in Document 5 that, if the difference in thickness between the two ferromagnetic layers increases, the effect of antiparallel coupling decreases, making it difficult to control the magnetization distribution in the recording layer.
In the recording layer having such a stacked structure, it is difficult to suppress the increase in switching field due to the miniaturization of the element and to control the magnetization distribution in the recording layer.